Method for measuring the relative extent of burnout of combustion elements in a pebble-bed high-temperature reactor (htr) and a corresponding device

ABSTRACT

Previous methods for measuring the burn-out of a combustion element from a pebble-bed high-temperature reactor, in particular with high circulation rates, have as a rule an error margin of up to 10%. The inventive method for measuring burn-out is extremely rapid, but nevertheless highly accurate, with an error margin of only 1-2%. The method comprises the following steps: a combustion element is removed from the reactor and transferred to a measuring position; b) the combustion element is subjected to a thermal neutron flux; c) a first detector determines the γ radiation emitted by the combustion element; d) if the measured value exceeds a previously defined first threshold value, the combustion element is fed directly to the reactor, but if the measured value falls below the threshold value, the combustion element is subjected to steps e to f; e) a second detector determines the high-energy radiation above 1 MeV emitted from the combustion element; f) if the measured value exceeds a previously defined second threshold, the combustion element is fed to the reactor, but if the measured value falls below said threshold value, the combustion element is evacuated from the combustion element circuit.

The invention relates to a measurement method for fuel elements,especially a measurement method with the aid of which the burnout of thefuel elements in a pebble-bed high-temperature reactor can bedetermined.

STATE OF THE ART

In the operation of a pebble-bed HTR with multiple passes (like AVR orTHTR), a certain proportion of the recirculated fuel elements (FE) mustbe removed from the circulation to provide place for the addition offresh fuel elements. It is thus naturally in the interest of goodfissionable material economy to remove the fuel elements which haveburned out to the greatest extent where possible. For this purpose eachindividual circulated fuel element is subjected to a measurement. Whatis measured is a physical parameter which constitutes a measurement ofthe degree of burn-out. It is important in such a system, in theinterest of good measurement precision, not necessarily that there be aproportionality between this parameter and the degree of burn-out, butrather a greater measurement effectiveness and good reproducibility ofthe parameter which is measured. Based upon the measured parameter, adetermination is made as to whether the fuel element is to be fed backto the reactor core and optionally to which zone of the reactor core itis to be fed, or whether it is to be removed.

Within the reactor core a fission process is carried out as a result ofwhich fission products are produced by the fissionable material withinthe fuel elements. The individual fuel elements during the circulationare located outside the reactor core in the ball or pebble removal tubeso that further fission processes are suppressed. The fission productswithin the fuel elements are however radioactive and emit on their partγ radiation (gamma radiation). For different fuel elements, the measuredtotal γ radiation emitted from a fuel element under substantiallyidentical conditions, for example the same duration after emergence ofthe fuel element from the reactor core, is correlated with its burn-out.

Up to now various measuring processes have been used to determine thedegree of burn-out of ball-shaped fuel elements.

With the AVR (Working Group Test Reactor), because of the relativelysmall circulation velocity of the fuel elements of about 500 per day, aγ spectrometric measurement of the Cs¹³⁷ present in the fuel element ispossible with a liquid-nitrogen-cooled semiconductor detector. Thesemeasurements are only somewhat expensive and supply over acceptablemeasurement times of 20 to 40 seconds a measurement precision in therange of ±2% for highly burned-out fuel elements.

With modern modular pebble-bed power reactors, like the HTR module ofSiemens or the South African PBMR, the circulation speed is much higherby comparison with the AVR (about 4000 fuel elements per day) and thedecay time of the fuel elements in the ball withdrawal tube isrelatively short (about 2 days) so a direct translation of the measuringprocess from the AVR to the higher speed circulation of such reactors isnot possible if only because of the short measuring duration which isavailable. A shorter measurement time invariably gives rise to greatermeasurement error. Of greater significance is the fact that because ofthe very short decay time of the fuel elements, the evaluation of theCs¹³⁷ line can be very imprecise. The high activity of the short-livedfission products is particularly detrimental as far as the γ measurementof the Cs¹³⁷ is concerned since the evaluation of the typical 662 keV ofthe Cs¹³⁷ is significantly influenced by the neighboring lines. Amongthese are the very strong 658 keV line of Nb⁹⁷ (effective half life=16.8hours), the weaker 661 keV line of Ba¹⁴⁰ (half life 12.8 days) and thestrong 668 keV line of the I¹³² (effective half life 76.3 hours). Acorresponding correction of the measured Cs¹³⁷ signal as a rule wouldrequire very expensive measurement technology to carry out. The fastcirculation in combination with a short ball discharge tube and thus ashort residence time in the ball discharge tube can thus give rise to asignificant influence on the reproducibility of the Cs measurement.Concrete tests of an actual reactor are not however available as yet.Those skilled in the art have treated the attainable precision verydifferently. Generally however it has been believed that with highlyburned-out fuel elements, it is not possible to do better than a meanmeasurement error of ±10%.

In corresponding expert circles, alternatives have been proposed for thesimple measurement of total γ activity of fuel elements for modernmodular pebble-bed power reactors.

The γ activity of an irradiated fuel element is dominated in the reactorcore and even after its emergence from the core in the case of a not toogreat decay time by the short-lived fission products. The contributionof the longer-life fission product to the intensity of the radiation ispractically negligible. Fuel elements which have been burned out to alesser extent have, in the reactor core and thus also shortly beforetheir emergence from the core, a greater power production or powerproductivity than more burned-out fuel elements and thus also a higher(short lived) γ activity. The measurement effect in terms of thedifference in γ radiation between a fuel element which has been burnedout to a lesser extent and a fuel element which is highly burned out orburned out to a greater extent is very high. (In the case of the AVRwith its comparatively long decay time of the fuel elements of anaverage of say one month, the γ activity of the fuel elements burned outto a lesser extent is always about 3 to 4 times higher than the γactivity of the highly burned-out fuel elements. These methods areindeed not very accurate although they are very easily carried out andunusually fast (measurement time about 1 second).

As state of the art, today, the combination of measurements of the totalγ activity and of the Cs¹³⁷ radiation can be recognized. All of the fuelelements are thus subjected to a simple γ measurement (for example 1second). Only with fuel elements which have been recognized as highlyburned-out fuel elements is the γ activity value undertaken below theaforedescribed limits as a parallel Cs¹³⁷ measurement (about 10seconds). Only after the evaluation of the Cs¹³⁷ measurement is adetermination made as to whether the fuel element is recirculated orwithdrawn.

However, even with this combination method which permits the longermeasurement duration for the Cs measurement as a rule, large mean errorshave to be reckoned with which fuel elements which are burned out to ahigh extent. The experts in the field have indicated that the precisionattainable is from ±4% to ±20%.

OBJECTS AND SOLUTION

The object of the invention is to provide a measurement method forball-shaped fuel elements which with a shorter decay time of the fuelelement and in a brief measurement time or duration during thecirculating operation of a pebble-bed reactor can determine the degreeof burn-out of a field element.

It is an object of the invention further to provide a correspondingdevice for carrying out the above-mentioned measurement method.

The objects of the invention are achieved by a method for determiningthe degree of burnout of a ball-shaped fuel element according to themain claim and an apparatus for carrying out the method according to theauxiliary claim. Advantageous embodiments of the method and theapparatus are to be found in the claims respectively dependent thereon.

SUBJECT OF THE INVENTION

The invention is not a measurement process for 20 determining theabsolute value of the burnout (for example in % FIMA=Fission per initialmetal atom) of a ball-shaped fuel element. The invention is also notprovided to determine the burnout for fuel elements which have only alow degree of burnout. For these fuel elements, it is possible todetermine the burnout based upon their significantly higher γ activityby a simple γ measurement.

The new and inventive method is especially provided to determine thedegree of burnout for such fuel elements which have a higher degree ofburnout than that which can be determined by a simple γ measurement,especially in conjunction with the possible removal thereof as will bemore specifically defined below.

The subject of the invention therefore is a method of measuring thedegree of burnout of a ball-shaped fuel element which is similar to theaforedescribed process which has been determined as the combinationmethod. The fuel elements gated out of the reactor core are subjected toa brief simple γ measurement. Based upon a previously established firstlimiting value or threshold of the γ activity, the fuel elements whichare subjected to measurement are subdivided into lower burnout or higherburnout fuel elements. The fuel elements which are recognized as havinga higher degree of burnout are subjected to a further measurement. Thissecond measurement is based upon the presumption that in a fuel elementwhich is excited or activated with thermal neutrons more fission eventsoccur as the degree of burnout is less, i. e., the greater the number offission events, the smaller the degree of burn out. During a fissionprocess, hard γ radiation is spontaneously emitted. The intensity of thehard γ radiation, especially above 2 MeV, can thus form a measurement ofthe degree of burnout of the fuel element.

The measurement method is carried out as follows: The fuel element ballsare gated out of the reactor core, for example in the course ofcirculation, and transferred to a measurement position. There they aresubjected to a thermal neutron flux which gives rise to nuclear fissionwithin the fuel element. Apart from the γ activity of the fissionproduct already present, this leads to the emission of the so-calledspontaneous radiation which results from the nuclear fission and is inaddition to that γ activity and in the form of a hard γ radiation. Onaverage this hard γ radiation is more energy rich, i.e. of a higherenergy, than the γ radiation of the fission products.

In a first measurement step, the total γ activity of the fuel element ismeasured with a first defector. This measurement is carried-outtypically very quickly (about 1 second) which however is not veryprecise. This measurement serves only as a first estimation of thedegree of burnout of the fuel element investigated. For a given reactor,the probability that a fuel element will have a certain overall γactivity complies with a statistical probability distribution. This isdependent, among other things, upon the point in time at which the fuelelement is measured after discharge from the reactor core. Fuel elementswith a high degree of burnout have only few fission products so that theactivity of the γ radiation emitted by these products is small. If onehas a fixed upper limit for the γ radiation at which, upon a γ radiationmeasurement above this limit, the fuel element will be returned to thereactor core, one can make a preselection of fuel elements for which afurther measurement may be anticipated. The threshold can be determinedbased upon the probability distribution. For example the threshold canbe selected at the point at which a maximum of 20% of all of themeasured fuel elements have their measured activity lying below thethreshold. Only for these 20% will the second measurement be anticipatedand this second measurement can advantageously be carried out inparallel with the first. The second measurement step of the methodaccording to the invention resides in the provision of an appropriatesecond detector which enables only a hard γ radiation of a fuel elementto be detected. The method according to the invention thus utilizesadvantageously the reactor which is present and from which the fuelelements are removed as the neutron source to produce in the fuelelement, nuclear fission. The detector suitable for this purpose mustespecially be capable of detecting the energy-rich radiation whichpreferably lies above 2 MeV. For this energy selective measurement, theenergy revolution of a NaI scintillation counter is for examplesufficient. The second detector should operate with a γ total pulse rateof greater than 10⁷/second, especially greater than 10⁸/s %,

Because of the short decay time of the fission products within the fuelelements, the γ activity of the fission products dominates the γactivity of the fission clearly. Many solutions can be used individuallyor in combination since the useful signals of the hard γ radiation isnot nearly as significant as the greater activity of the not soenergy-rich γ activity of the fission products of the fuel element onwhich it is superimposed.

1. It is advantageous to use a detector for the second measurement stepwhich operates at a very high pulse rate, that is which has a very goodtime resolution and thus in a brief measurement interval will only showminor errors.

2. Further, the ratio of energy-rich (hard) radiation to the not so richγ radiation, favors the hard radiation if between the detector and fuelelement a shield is provided which operates effectively with a high passenergy filter and thus weakens the lower energetic γ radiation impingingon the second detector. Such a shield can be formed, for example, by alead filter.

3. Since the second measurement is carried out with especially highprecision, the second detector can advantageously be so arranged thatits optimum working range lies at radiation values which are exactlythose which are emitted by the fuel elements of interest, namely, thosewhich are more highly burned out. This means that the radiation valuesfor the lower degree of burnout fuel elements are significantly higherthan the optimal working range of the second detector. To preventpossible damage to the second detector, however, several steps can betaken. Especially additional appropriate shielding can be provided sothat during the measurement of fuel elements of a lesser degree ofburnout, the second detector will not be overloaded. Alternatively,however, the second detector can be deactivated during measurements offuel elements with a lesser degree of burnout, something which can bedone especially simply in the case of successive measurements 1 and 2.

4. The number of induced nuclear fissions within the fuel elementincreases with increasing neutron flux (measurement flux). For producingthe highest possible thermal neutron flux at the measurement position ofthe fuel element, it is suitable especially to use the reactor coreitself (from which the fuel element derives) as a neutron source. Inprinciple, however, also other neutron sources are suitable.

5. The measurement position is advantageously surrounded by water. Inthis manner the reactivity of the subcritical measurement arrangement isincreased and fission-liberated neutrons utilize as much as possible forthe further fission events. The fuel elements subjected to measurementthemselves, with their fissionable material, influence the reactivity ofthe arrangement.

This results in an amplification of the measurement effect.

6. To increase the precision of the second measurement, alternatively aplurality of the second detectors can also be provided and can haveparallel counts which can be added.

7. Further, a plurality of fuel elements can be measured at severalmeasurement positions simultaneously and in parallel. So as not tochange the recycling rate of the circulation, in this manner a greatermeasurement duration is made available for each measurement with as arule a positive effect on the precision of the measurements.

8. Basically one can also increase the time between the discharge of afuel element from the reactor core and its measurement (intermediateduration) advantageously, since the γ radiation of the fission productsdecreases with time in accordance with their decays although the γradiation from the induced fission remains unaffected thereby. Thishowever requires disadvantageously expensive structural changes orunsatisfactory reactor operations.

The method of the invention allows in a simple manner a highly preciseindication (error rates of only about 1 to 2%) as to the degree ofburnout of a fuel element to be obtained. This method is especiallysuitable for distinguishing whether a fuel element circulated in ahigh-temperature reactor (HTR) should be gated out or returned to thereactor core. The method supports this determination advantageously asfollows:

a) a fuel element is removed from the reactor and transferred to ameasurement position,

b) the fuel element is subjected to a thermal neutron flux,

c) a first detector detects the γ radiation emitted from the fuelelement,

d) upon the measurement exceeding a predetermined first threshold value,the fuel element is directly recycled again to the reactor and upon themeasurement falling below the limiting value or threshold, the fuelelement is processed by the steps e to f,

e) a second detector determines the high energy γ radiation above 1 MeVemitted from the fuel element,

f) upon exceeding a predetermined second threshold or limiting value bythe measurement, the fuel element is recycled to the reactor and uponthe measured value lying below this threshold, the fuel element is gatedout of the fuel element circulation.

SPECIAL DESCRIPTION PART

In the following the subject matter of the invention will be describedin greater detail in connection with an embodiment and a Figure withoutthe subject matter of the invention being limited thereto.

The Figure shows in horizontal section an embodiment of a device forcarrying out the method of the invention. In that Figure the referencecharacters have the following significance:

-   -   1 Reactor, outer side of the biological shield    -   2 Thermal column (graphite) with thermal neutron flux    -   3 Ball tube    -   4 Water tank    -   5 Biological shield    -   6 Fuel element in measurement position    -   7 Plug for detector replacement    -   8 Energy selective second γ detector with higher time resolution    -   9 Connecting cable to pulse processing    -   10 Detector shielding and energy filter, for example of lead    -   11 First γ detector    -   12 Circulating apparatus (shown schematically) with fixing of a        fuel element in the measurement position

The method of the invention is then carried out as follows in thedevice:

The fuel element 6 to be measured which has been gated out of thereactor core, is brought into a defined measurement position 12 in whichit is subjected to a thermal neutron flux 2. Depending upon the degreeof burnout or the fissionable material still contained in the fuelelement 6, nuclear fission events occur in the interior of the fuelelement with an intensity which is determined by measurement. Themeasured value thus represents the hard, high-energy γ radiation whichis produced from the fission products thermally after the fission eventsand is emitted by the fuel element (spontaneous radiation). Themeasurement uses the fact that the energy of this hard γ radiation onenergy is higher than the γ radiation emitted or by the fission productspreviously present in the fuel element. As a consequence, the harderenergy-rich γ radiation is detected by an energy selective γ measuringdevice. A suitable detection system is for example a high-resolutionscintillation counter 8 with high time resolution whose energyresolution is sufficient for the purpose.

The smaller portion of the higher energy γ activity from the fissionproducts in the fuel element which fall in the range of the hard γradiation to be measured can be measured therewith without a significanteffect on the measurement precision since the total γ radiation of thefuel element to be measured is also dependent upon the degree of burnoutand, indeed, in the same way. The higher the burnout the less thefission product content and thus the fission effect on the measurementand the smaller the level of the hard γ radiation and the smaller alsothe total fuel element activity (as to the possibility of a simplemeasurement of the total γ radiation of a fuel element as an effectivemeasurement process, reference is made to the foregoing discussion onthe point).

The two features which have been described, therefore, characterize theprinciple of the new method. The main difficulty is that because of theonly short decay time of the FE (typically 2 days) the γ activity isvery high (noise signal) and compared to it the hard γ radiation as auseful signal is completely in the background. If in spite of this theabove-described precision is to be achieved, that is that in a shortmeasurement interval a sufficiently large number of useful pulses are tobe accumulated, the following further features of the method take ongreat importance.

The γ measurement device 8 (second detector) is used which can operatewith a very high pulse rate and this has very good time resolution. Theshielding 10 between the measurement device 8 and the fuel element 6 tobe measured is so configured that the measurement device in the case ofa fuel element burned out to a greater extent, that is a fuel elementwhich is comparatively weakly emitting radiation already operates in theregion of its maximum possible count rate. Highly radiating fuelelements which have been insufficiently burned out can no longer bedetected with this second detector. These however in the sense of thealready described combination method will be recognized with the aid ofthe simple γ measurement using the first detector 11.

The requisite shielding 10 between the fuel element 6 and the measuringdevice 8 (second detector) is made from lead so as to have the greatestpossible energy filtration effect (preferably allowing only the hard γradiation to pass).

The consumption of fissionable material for the measurement iscompletely negligible as a result of the short measurement duration evenwith very high neutron flux (measurement flux). In the interest of goodmeasurement precision, the method can operate with the highest possiblemeasurement flux. For that no external neutron source is applied butrather the reactor core itself is advantageously used as the neutronsupplier. For that purpose the reactor contains, like with researchreactors such as, for example, the Dido reactor of Forschungszentrum J

lich GmbH, a “thermal column” 2 that is a throughgoing graphiteconnection between the side reflector and the outer side of thebiological shield extending radially and interrupting the reactorvessel, if possible, at the level of the center of the reactor core.Directly ahead of the outer end surface of the graphite, the measurementposition 12 is located. This measurement position 12 is in additionadvantageously surrounded by water. As a result the reactivity of thesubcritical measuring device is increased and the fission-liberatedneutrons are used to the greatest extent possible for further fissionevents. The fuel elements to be measured themselves influence with theirfission-material content the reactivity of the device. This leads to anamplification of the measurement effect.

In a practical embodiment the measurement position 12 is effectivelyprovided in a part of the ball-charging apparatus 3 in which the ballscan be retained without additional means. For this purpose, aball-feeding deflector or switch s provided which can control the targetwith respect to the measured ball, i.e. whether the ball is returned orfed to the pebble bed or the ball is discharged. The arrangement of thefeed deflector at the level of the reactor core middle (ahead of the“thermal column”) affords the advantage that the long path of the ballfrom the region of the lower end of the ball withdrawal tube to itsupper point of direction reversal of the feed tube or feed tubes to thepebble bed is subdivided into two partial stretches and thus theindividual pneumatic ball conveyor path can operate with less gasdisplacement pressure and volume.

The entire measuring arrangement is also provided with a biologicalshield 5 which surrounds the measuring arrangement and a further γdetector 11. This detector is so arranged that it operates at a highercount rate when a fuel element with a smaller degree of burnout (forexample after one pass through the core) is found in the measuringposition 12. With this detector 11, all of the fuel elements gated intothe measurement position (and thus the balls) are measured in the senseof the above-described combination method with respect to their γactivities. If the measurement results of this detector level exceeds acertain predetermined limiting value or threshold, a measured fuelelement is not yet sufficiently burned out and is recycled withoutwaiting for the second measurement to the reactor core. If themeasurement by the detector 11 lies below the limiting value orthreshold, the fuel element is subjected to a measurement also by thedetector 8 and a determination is then made with respect to the balltarget, namely is the withdrawal or recycle. This choice is made againby comparison of the measurement result with a further limiting value orthreshold. When the measurement lies beneath the limiting or threshold,the fuel element is withdrawn.

The two limiting values can be determined from the probabilitydistribution of the measurement results of a large number of previouslymeasured fuel elements, for example 300. This number is equal to thearea beneath the distribution curve. For the limiting value or thresholddetermination, such values are selected on the measurement value scalewhich divide the area under the curve into certain predetermined countratios. If for example 20% of all measured fuel elements also areintended to be measured with the second detector, the distribution curvearea will break up the measurement results from the first detector 11 ina ratio of 2:8. In that case 20% of all of the measurement results willlie below the first threshold or limiting value.

If it is assumed further that the reactor is operated in a 1:10 mode,that is that for each freshly fed fuel element, 10 are recycled and alsoon a long-term basis that for every 10 recirculated fuel elements, onefuel element must be removed, the 10 proportion removed must equal 10%so that in the probability distribution, the measurement result of thesecond detector 8 should provide a value which will divide the areaunder the distribution curve into two equal halves. The fuel elementswhose measurement results thus fall below the second threshold orlimiting value are to be removed. The removal proportion is then 10%which corresponds to the 1:10 operating mode. The probabilitydistribution and thus also the limiting value and threshold calculationcan reflect the realities of the fuel element measurements. When thereis a change in the reactor power, the measurement results beforeprocessing are multiplied by the ratio between the newer and earlierpowers.

Should the second detector 10 in its switched-on state be subjected tosupersaturation with γ radiation to the point that it might be damaged,then advantageously both measurements would not be carried out inparallel and initially only the γ radiation measured by the firstdetector 11. Only when the measurement results lie below the firstthreshold or limiting value would the operating voltage for example ofthe second detector 8 be turned on. It can be noted further that themeasurement need not be carried out as shown in the drawing ahead of thebiological shield 1. It can also be carried out in a pocket formed inthe biological shield. As a result, the thermal column is shortened andthe measurement flux 2 is greater. It is also possible to carry out themeasurement directly at the outer side of the reactor pressure vessel.The second detector is then naturally located outside the γ radiation ofthe reactor core to a more significant extent. Since the highermeasurement flux 2 is so significant however for the precision of themethod that in this case a constant measurement of the γ background mustbe taken into consideration as long as it is not dominant.

1. A method of measuring a relative magnitude of the burnout of a fuelelement with the steps a) a fuel element 6 is removed from the reactorand transferred to a measuring position 12, b) the fuel element 6 issubjected to a neutron flux 2, c) a detector 8 determines a relativemagnitude for the high energy γ radiation above 1 MeV as emitted fromthe fuel element.
 2. The method according to claim 1 in which prior tostep c) a first detector 11 determines a relative magnitude of the γradiation emitted from the fuel element.
 3. The method according toclaim 2 in which in step c) it is first determined if the measured valueof the γ radiation detected by the first detector 11 lies below apredetermined first limiting value or threshold.
 4. The method accordingto claim 1 in which the second detector determines a relative magnitudefor the high energy γ radiation above 2 MeV emitted by the fuel element.5. The method according to claim 1 in which a second detector 8 with acount rate of at least 10⁷/s, especially at least 10⁸/s, is provided. 6.The method according to claim 1 in which a scintillation counter is usedas the detector
 8. 7. The method according to claim 1 in which betweenthe measurement position 12 and the detector 8 a shield is provided. 8.The method according to claim 7 wherein a lead filter is provided asshielding between the measurement position 12 and the detector
 8. 9. Themethod according to claim 1 in which the first detector 11 detects the γradiation of the fuel element 6 in less than 2 seconds.
 10. The methodaccording to claim 1 in which the second detector 8 detects the highenergy γ radiation of the fuel element 6 in less than 30 seconds,especially in less than 10 seconds.
 11. The method according to claim 1in which he fuel element 6 in the measurement position is surroundedwith water.
 12. The method according to claim 3 in which the firstlimiting value or threshold for the first γ measurement is so selectedthat a proportion of the fuel elements which are required for operatingthe reactor falls below this limiting value.
 13. The method according toclaim 3 in which the first limiting value for the first γ measurement isso selected that with a 1:10 mode of operating the reactor a maximum of20%, especially a maximum of 15% of the fuel elements lies below thislimiting value.
 14. The method according to claim 1 in which the fuelelement 6 is gated out of the reactor circulation when the measuredvalue of the high-energy γ radiation detected by eh detector 8 liesbelow a predetermined second limiting value.
 15. The method according toclaim 14 in which the second limiting value for the second measurementis so established that a proportion of all fuel elements which aremeasured but are required for operating the reactor falls below thislimiting value.
 16. The method according to claim 13 in which the secondlimiting value is so established for the second measurement that in a1:10 mode a maximum of 15% and especially a maximum of 10% of allmeasured fuel elements lie below this limiting value.
 17. A device forcarrying out the method according to claim 1 having a) a neutron sourcewhich generates a thermal neutron flux 2, b) a measurement position 12for fixing a fuel element to be measured so that he fuel elementundergoes the thermal neutron flux, and c) a detector 8 which is capableof measuring high energy γ radiation 6 emitted from the fuel element 6arranged at the measurement position.
 18. The device according to claim17 with an additional first detector 11 which detects γ radiation 6emitted by the fuel element 6 at the measurement position.
 19. Thedevice according to claim 17 with a shield 10 between the measurementposition 12 and the second detector
 8. 20. The device according to claim19 where a shield 10 is a lead filter.
 21. The device according to claim17 with a scintillation counter as the second detector.
 22. The deviceaccording to claims claim 17 with a second detector having a countingrate of at least 10⁷/s, especially at least 10⁸/s.
 23. The deviceaccording to claim 17 in which the measurement position 12 is at leastpartly surrounded by water.